> Is there anything in the firmware or in rockbox that prevents me
> from
> using a 120GB disk?

Your budget, perhaps, they are unreasonably more expensive than 100 GB. You
pay a premium for the last 20 GB.
Technically no problem below 137 GB (drive manufacturer style of counting).
>From then on we'd need BigLBA, something the USB bridge is incapable of.

> What are the largest disks you folks use? (Mine has
> currently a 40 GB disk, and it's full...)

We have several 100GB users, afaict nobody with 120 yet. But I'm also eying
on that, have a Fujitsu 4800 rpm drive in focus.